The intensive development in fields of technology such as electronics, biology, and material science is a strong force towards stimulating nanotechnology in general and the synthesis of nanomaterials in particular. The driving force that pushes research toward the investigation of materials at the nanometer and atomic scale is linked to the need, in modern technology, of miniaturizing systems and devices and of dramatically increasing their efficiency.
There is a further need for simple, economically feasible and reliable techniques for the preparation of functional nanoscale periodic and aperiodic patterns on surfaces, for instance, in order to provide selective seeds for electroless deposition of Cu during fabrication of printed circuits and boards. In the widely applied current method the conductive thin layer of copper is deposited onto the dielectric substrate and generally in order to plate on the substrate it must be seeded or equipped with special catalyst prior to deposition of metal. By the established method the selective formation of metallic particles (seeds) requires many steps and therefore is complicated and expensive.
Another promising application of nanostructured surfaces is the production of functional nanoscale devices, such as quantum dot transistors, where the data are transmitted not by currents, like in traditional technologies, but single electrons which jump from one quantum dot to another. This is of great importance in research activities on single-electron devices (SEDs), where the driving force is the limitation in trend towards reducing the size while increasing the complexity of traditional chips.
Nanoarrays of the clusters can be used in “bio”-applications such as decoration of the surfaces with bio-molecules. Here, a patterned surface where molecularly well-defined adhesive spots are separated by the desired non-adhesive regions can provide special binding groups for selective proteins. The high precision in construction such binding sites allows to mimic nature, which very selectively binds molecules to their receptors in a way as key fits into lock. This is of high interest in fabrication of nanosensors, bio-chips, high density data storage devices and other applications from the border of biology and nano-electronics.
The surface selectively decorated with metal spots has great importance for the optical properties used in sensors and analytical devices such as Surface Enhanced Raman Spectroscopy and Imaging Ellipsometry, where in order to achieve highest quality and reliable results the spots should possess homogeneous density, the same shape and size.
The fabrication of devices with nanometer precision can be achieved following two different approaches: the top-down approach which scales down devices to the nanometer level, and the bottom-up approach which uses certain strategies to obtain building blocks for the construction of nanometer devices.
The “top-down” approach is based on a variety of different lithographic techniques like the well-established photolithography, ion-beam, electron- or X-ray lithography. These methods allow the fabrication of periodic and aperiodic patterns with good resolution down to 100 nm (optical lithography) or even 10 nm (e-beam lithography). However, as lithography moves to shorter wavelengths, the associated costs of the process increase very rapidly. Also different unconventional scanning probe lithography methods such as “dip pen” nanolithography (DPN) can be used to selectively decorate surfaces with nano-objects. The dimensions accessible with these techniques fulfill most of the current requirements for electronic and biological applications.
The “bottom-up” approach exploits the self-assembly of molecules into nanostructures. Molecules such as organic conjugated-molecules, surfactants or block copolymers can self-organize into periodic patterns with the resolution of few nanometers. The pattern is controlled by the “chemistry” of the molecule and can be designed by the proper synthetic pathway that leads to appropriate structures of the molecule and in consequence to desired patterns. Block copolymers can form a rich variety of nanoscale periodic patterns. The morphology of obtained nanoscale structures and their periodicities depend on molecular weight, the strength of interaction between the blocks (represented by the Flory-Huggins interaction parameter, χ) and the volume fraction of one of the constituent blocks.
In EP 1 027 157 B1 there is disclosed that metal nanoparticles can be patterned at the surface using the self-organisation of block copolymers to core-shell systems (e.g. micellar structures) which act as nanoreactor in particle formation. In the method disclosed therein, for example, an amphiphilic poly(styrene)-b-poly(2-vinylpyridine) polymer is dissolved in a selective solvent and reverse micelles are created, where PS blocks form corona and P2VP core of the micelles. Addition of HAuCl4 to the system results in complexation of inorganic salt by the core of the micelle. The loaded micelles are transformed onto the surface of the substrate in a form of regular pattern. Subsequently the polymer matrix is removed, for example by gas plasma treatment. At the same time metal nanoparticles are produced by the reduction of the precursor salt. The thus obtained single Au clusters are deposited on the surface in a pattern reflecting the previous micellar arrangement. The size of the cluster can be adjusted in the range from 1 to 15 nm, while the interparticle distance can reach >200 nm. The method of preparation ordered nanoparticles as described in EP 1 027 157 B1 is not limited to the exemplary system HAuCl4 in PS-b-P2VP micelles, but a large variety of inorganic precursor salts and block copolymers can be chosen as well as various substrate materials.
Self-organization is a very powerful way of obtaining nanoscale patterns, however it is hard to prepare non-periodic structures in this way.
Aperiodic structures of nanoscale ordered nano-objects can be achieved by combining self-organization of the macromolecules on the nanometer scale and fabrication of large scale structures by “top-down” technique.
An example of such combination is the microcontact printing (μCP) of self-assembled monolayers on substrates. Poly(dimethylsiloxane) (PDMS) stamps are molded on masters fabricated by photolithography or e-beam lithography. A thus prepared stamp is then used to transfer molecules of the “ink” to the surface of the substrate by contact. However, when applied at the nanoscale, μCP remains a more significant challenge than producing micrometer scale patterns. Two key factors that determine the limits of resolution are lateral diffusion of the molecules and distortions of the stamp. Nevertheless, lateral dimensions as small as 50 nm can be achieved.
Recently, two methods for the realization of periodic and aperiodic patterns of core-shell systems on substrates have been proposed. One method combines e-beam or photosensitive resist pre-structuring with self-assembly of the core-shell systems such as for example block copolymer micelles; cf. EP 1 244 938 B1. In this method the template is formed on the e-beam resist by writing a pattern by e-beam lithography and further developing written areas. Onto the resist which possess patterned lines, loaded micelles are deposited for example by spin-coating. During the evaporation process, due to capillary forces micelles move inside the groves. Next, the resist is dissolved in an appropriate solvent and only micelles with direct contact with the substrate remain. The polymer matrix is removed and the precursor salt is reduced to the noble metal for instance by a gas plasma process. This approach can be used for the arrangement of nanoclusters in a variety of patterns, for example 7 nm wide lines, separated by 500 nm. This technique allows the positioning of particles with nanometer size in periodic and non-periodic patterns, in contradiction to traditional lithographic approaches which are not able to write structures in nanometer dimensions over large micrometer areas, whereas self-assembly methods fail in positioning particles in artificial patterns with large separation distances.
The second proposed method is based on direct e-beam writing of monomicellar films which function as negative resist; cf. R. Glass et al., Adv. Funct. Mater. 2003, 13, 569. A flat substrate is covered with the monolayer of loaded micelles, assembled into highly ordered periodic structures. Areas of the micellar monolayer are directly exposed to the electron beam that modifies the polymer chemically. Non-irradiated micelles are removed from the substrate by the lift-off procedure using an appropriate solvent. A final treatment leaves pattern of gold nanodots on the surface, removing all organic material. Squares, circles and rings, each consisting of nanoclusters with uniform size and interparticle distance, ranging from a few nanometers up to several micrometers can be produced by this technique. The e-beam writing on micellar monofilms requires very precise electron dose and time of irradiation. Any deviation in electron exposure results in not fully crosslinking of the polymer (in consequence lifting-off also irradiated micelles) or partially reduction of the metal leading to broad size distribution of the micelles and destruction of short-range order between the particles.
Both approaches as discussed hereinabove, however, require very expensive equipment or high energy doses, respectively. Further, they are time-consuming serial processes not suitable for patterning of large areas. Moreover, they are not suitable for non-conductive substrates, unless additional treatment is carried out.
Accordingly, there still exists a strong need for a more simple and efficient method for manufacturing nanometrically surface-decorated substrates without the shortcomings mentioned above. Thus, it is an object of the present invention to provide an economic method for manufacturing nanometrically surface-decorated substrates, which allows decoration of large areas with periodic and aperiodic patterns of nano-objects, with good control over two different length scales: nano- and micrometers.